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An Overview of Tungsten Toxicity
Published in Debasis Bagchi, Manashi Bagchi, Metal Toxicology Handbook, 2020
Ola Wasel, Jennifer L. Freeman
Tungsten (W) is a transition metal with an atomic number of 74, a molecular weight of 183.84, and belongs to Group VIB of the periodic table. Tungsten is present naturally in rocks and minerals. Tungsten is not present in a pure form, but it is naturally combined with other metals.1 The most common forms of tungsten that are used in industrial applications are wolframite and scheelite.1 Tungsten has the highest melting point and highest tensile strength at a temperature of over 1,665°C compared to all other metals.2 Tungsten has several oxidation states: 0, +2, +3, +4, +5, and +6. The physical and chemical properties of tungsten compounds vary based on the oxidation state (Table 24.1). Tungsten is used in the forms of tungsten carbide, metallic tungsten, tungsten chemicals, and tungsten alloy in many different applications (Figure 24.1).1,3
Isotope geochemistry of Los Santos (Spanish Central System) calcic scheelite skarn: constraints on the source of the fluids and tungsten
Published in Adam Piestrzyński, Mineral Deposits at the Beginning of the 21st Century, 2001
F. Tornos, C. Galindo, B.F. Spiro
The mineralization consists almost exclusively of scheelite. It occurs as coarse grains related to the clinopyroxene in both the endo- and exoskarn but also is abundant as fine-grained crystals in the amphibolitic skarn and especially in the biotitic skarn that can host the highest grades of the orebody. The sulphide content is very low, with the sulphides restricted to some irregular zones. The dominant as semblage includes arsenopyrite, pyrrhotite, chalcopyrite and pyrite. The Sn content is also very low (<100 ppm; Crespo et al., 2000).
Bismuth Vanadate Based Nanostructured and Nanocomposite Photocatalyst Materials for Water Splitting Application
Published in Mahmood Aliofkhazraei, Advances in Nanostructured Composites, 2019
S. Moscow, K. Jothivenkatachalam
Recently, Bismuth Vanadate (BiVO4) has gained increasing attention for its use as a promising candidate under visible light irradiation among the bismuth metal oxide photocatalyst (Pilli et al. 2011, Moscow and Jothivenkatachalam 2016). The BiVO4 photocatalysts are highly promising for different applications such as renewable energy production systems (i.e., solar fuels production from water and sunlight) and to resolve environmental issues. Bismuth vanadate (BiVO4), which is an n-type semiconductor, has been identified as one of the most promising photocatalytic materials. As it is well known, BiVO4 exists in three polymorphs of monoclinic scheelite, tetragonal scheelite, and tetragonal zircon structures, with band gaps of 2.4, 2.34, and 2.9 eV, respectively. It is reported that BiVO4 mainly exists in three crystalline phases: monoclinic scheelite, tetragonal zircon and tetragonal scheelite structure (Lim et al. 1995, Bhattacharya et al. 1997, Luo et al. 2008) (Figure 2). Monoclinic scheelite BiVO4, (~ 2.3 eV band gap) shows both visible-light and UV absorption while tetragonal BiVO4 (~ 2.9 eV band gap) mainly possesses an UV absorption band. The UV absorption observed in both the tetragonal and monoclinic BiVO4 is associated with band transition from O2p to V3d, whereas visible light absorption is due to the transition from a valence band (VB) formed by Bi6s or a hybrid orbital of Bi6s and O2p to a conduction band (CB) of V3d (Ng et al. 2010). The scheelite structure can have a tetragonal crystal system (space group: I41/a with a = b = 5.1470 Å, c = 11.7216 Å) or a monoclinic crystal system (space group: I2/b with a = 5.1935 Å, b = 5.0898 Å, c = 11.6972 Å, and b = 90.3871), while the zircon-type structure has a tetragonal crystal system (space group: I41/a with a = b = 7.303 Å and c = 6.584 Å) (Park et al. 2013).
Selective Flotation of Scheelite from Calcite Using a Novel Reagent Scheme
Published in Mineral Processing and Extractive Metallurgy Review, 2022
Zhiyong Gao, Jian Deng, Wei Sun, Jianjun Wang, Yunfeng Liu, Fengping Xu, Qinghong Wang
Scheelite (CaWO4), a very important tungsten-bearing mineral, is usually separated from other coexisted calcium minerals such as apatite (Ca10(PO4)6F2), calcite (CaCO3), and fluorite (CaF2) using the froth flotation method (Bo et al. 2015; Gao et al. 2016; Hu and Xu 2003; Wang et al. 2016). However, as scheelite has similar Ca active sites and similar flotation response to conventional fatty acid collectors with other calcium-bearing minerals, especially calcite, it remains difficult to separate them selectively (Dong et al. 2019a; Gao et al. 2017a; Hu and Xu 2003; Rai et al. 2002; Zhang et al. 2020). The bad separation of scheelite and calcite decreases the grade of scheelite concentrate, which increases the cost of subsequent scheelite smelting and solid waste treatment (Ai, Liu and Zhang 2018; Martins and Amarante 2013; Sreenivas et al. 2004). Therefore, many researchers have attempted to separate scheelite from calcite using various depressants such as sodium silicate (SS) (Yin and Wang 2014), modified SS (Bo et al. 2015; Dong et al. 2018), sodium fluorosilicate (Dong et al. 2019b), sodium alginate (Chen et al. 2017a), calcium lignosulphonate (Chen et al. 2018a), dextran sulfate sodium (Chen et al. 2017b), sodium phytate (Chen et al. 2018b) and etc.